The present invention is in the field of considering the phenomena of slowly changing dimensions in the process of evaluating facts of or derived from a collection of facts organized as, or otherwise accessible according to, a dimensional data model. For shorthand throughout this description, such a collection of facts is referred to as a dimensionally-modeled fact collection.
It is known to respond to a query to a dimensionally-modeled fact collection by reporting on the facts contained in the dimensionally-modeled fact collection. Reports are typically generated to allow one to glean information from facts that are associated with locations in a dimensional data space according to which the dimensionally-modeled fact collection is modeled.
Locations in an n-dimensional data space are specified by n-tuples of coordinates, where each member of the tuple corresponds to one of the n dimensions. For example, (“San Francisco”, “Sep. 30, 2002”) may specify a location in a two-dimensional data space, where the dimensions are LOCATION and TIME. Coordinates need not be singled-grained entities. That is, coordinates of a single dimension may exist at, or be specified with respect to, various possible grains (levels of detail). In one example, a coordinate of a LOCATION dimension is comprised of the following grains: CONTINENT, COUNTRY, CITY.
The order of the grains may have some hierarchical significance. The grains are generally ordered such that finer grains are hierarchically “nested” inside coarser grains. Using the LOCATION dimension example, the CITY grain may be finer than the COUNTRY grain, and the COUNTRY grain may be finer than the CONTINENT grain. Where the order of the grains of a dimension has hierarchical significance, the value of a coordinate of that dimension, at a particular grain, is nominally such that the value of the coordinate of that dimension has only one value at any coarser grain for that dimension. In an example, a value of a coordinate of a LOCATION dimension may be specified at the CITY grain of the LOCATION dimension by the value “Los Angeles.” This same coordinate has only one value at the COUNTRY and CONTINENT grains: “US” and “NORTH AMERICA”, respectively.
There is a well-known phenomenon in the field of dimensional data modeling of “slowly changing dimensions,” mentioned briefly above. This is a phenomenon in which the relationship of grains for a dimension may change over time. While it may be contrived to consider the concept of slowly changing dimensions with reference to the example LOCATION dimension (since, generally, the relationship of CONTINENT, COUNTRY and CITY grains will not change over time), there are other more realistic examples of this phenomenon.
As one illustration, consider an EMPLOYEE dimension that is intended to represent an organizational chart of a company. In this example, the EMPLOYEE dimension comprises the following grains: ORGANIZATION, DIVISION, TEAM and PERSON. Using this example, it can be seen that values of coordinates at various grains may change as a person moves from one team to another team (or, perhaps, a team moves from one division to another division). For example, in one month, Joe worked on the Red Team; the next month, he worked on the Blue Team. This may be modeled by one EMPLOYEE dimension coordinate having the value “Joe” at grain PERSON and the value “Red Team” at grain TEAM, plus a second EMPLOYEE dimension coordinate also having the value “Joe” and grain PERSON but the value “Blue Team” at grain TEAM. It is also possible to encode in the representation of the dimension coordinates the specific time intervals during which these grain relationships obtained.